Today's communication networks are based on different techniques, both with regard to the radio access network and with regard to the core network. Some communication networks use only packet-switched, PS, techniques and some communication networks use circuit-switched, CS, techniques or both. Many UEs are multi-standard and support both CS and PS as well as different radio access techniques. For example, a UE may be configured to be operable in a Global System for Mobile communication, GSM, communication network, a General Packet Radio Service, GPRS, communication network, a UTRAN network and a E-UTRA network (also referred to as Long term Evolution, LTE, communication network). Such a UE being operable in both a circuit-switched communication network and a packet-switched communication network may then roam between different communication networks and be handed over between the different communication networks.
One example of a handover from a packet-switched, PS, communication network to a circuit-switched, CS, communication network is a Single Radio Voice Call Continuation, SRVCC, which is a PS-to-CS handover of a Voice over IP, VoIP, call.
For facilitating SRVCC of voice to the CS domain, the VoIP session needs to be IP Multimedia Subsystem, IMS, anchored. An SRVCC from the LTE network may be triggered due to different reasons, for example for load-sharing reasons or due to bad coverage. A typical example is that the SRVCC is triggered when the radio signal strength drops below a certain threshold of ‘bad coverage’. Thus, the SRVCC procedures make it possible for a voice bearer in a VoIP call to be continued even if LTE coverage is lost.
Speech interruption time is a key performance indicator, KPI, for the end user of the UE. In order to keep the speech interruption time low, one solution makes use of an Access Transfer Control Function, ATCF. This solution introduces the ATCF for session anchoring and an associated media anchoring functionality in an Access Transfer Gateway, ATGW. It is expected that a speech interruption time of slightly less than 300 ms can be achieved with this solution.
However, during a inter-Radio Access Type, RAT, handover procedure, i.e. a handover between different radio access networks employing different radio access technologies, for example second generation/third generation, 2G/3G, a handover of a speech call between GSM and Wideband Code Division Multiple Access, WCDMA, the speech is normally interrupted for less than 200 ms. This interruption time starts when the UE leaves the radio connection in the source network, and ends when the UE finds the radio resource in the target network and is synchronized to the new path. Such a short speech interruption time is not obtainable for SRVCC when performing a handover from fourth generation, 4G to 3G/2G, mainly because additional signalling is required to trigger the bearer to be switched at the anchoring point within IMS.